This invention relates to a method and apparatus for deaerating a low conductivity liquid. In a liquid coating process, the present invention employs electrostatic forces to deaerate the coating liquid prior to or after the coating step.
Deaerating, debubbling, defoaming, and degassing are commonly used to describe the same processes of removing a gas from a liquid. In this application, we define defoaming as removing surface bubbles, degassing as removing adsorbed gasses in fluids, creating bubbles, and growing existing bubbles to a larger size, and debubbling or deaerating as removing bubbles from a liquid, including from the surface. Vacuum and ultrasonic devices work by removing adsorbed gasses by forming tiny bubbles and growing existing bubbles to larger diameters. The bubbles are then brought to the surface using gravitational or centrifugal forces, with larger bubbles being removed more rapidly. In this application, we further define xe2x80x9cdeaeratingxe2x80x9d to mean facilitating the removal of bubbles of entrained gas from the liquid, by bringing those bubbles closer to the exposed surface of the liquid and rupturing them at that surface.
This invention has particular utility with respect to the deaeration of coating liquids, and especially for lower conductivity coating liquids, and even more especially for such coating liquids which have higher viscosities. Air entrapment and bubble formation is common in coating methods where the coating liquid is agitated or recirculated. Roll or gravure applicators which use open pans to supply liquid to the coating rolls are often hampered by air entrainment. Some coating liquids also have a tendency to contain or form a significant number of bubbles, especially those with high loadings of surfactants. In addition, the higher the viscosity of the coating liquid, the more difficult it is to remove any bubbles entrained therein. If a bubble gets caught at an interface during the coating process, such as between two coating rolls or in the gap between a die and a coating substrate, streaks and other defects in the coating can occur. In addition, if bubbles are present in the coating liquid dispensed from a curtain or slot coater, not only are defects in the coated products likely (such as bubbles or thin spots), but they may also cause the curtain or extrusion column to break. If air is trapped in the coating liquid as it enters into an oven, blisters and bubble defects will be seen in the dried product.
Known deaerating methods for coating liquids attempted to remove gas (e.g., air) from the coating solution prior to the coating step by holding the solution in a tank, by vacuum extraction, or by ultrasonic agitation. The use of large settling tanks inhibits quick changeover of the solution and increased waste thereof. Additionally, the higher the coating solution""s viscosity, the longer it""s necessary residence time (i.e., the longer it takes for the bubbles to rise to the surface). Vacuum methods of degassing and deaeration must maintain an effective seal against large vacuum levels, which is both costly and difficult to achieve. Ultrasonic methods are cost effective at degassing solutions by removing adsorbed air from the solution and growing larger bubbles in the solution that then more readily move to the solution""s surface due to gravitational forces, but are not as effective at breaking the bubbles at the surface of the coating solution.
In one embodiment, the present invention is a method of deaerating a gas contained in a low conductivity liquid. The method includes providing a support surface and establishing a layer of liquid having a first lower surface contacting a support surface and a second exposed surface. The layer of liquid has bubbles of gas entrained therein and has a thickness. The method further includes causing localized thinning of the layer of liquid by providing a flow of charged particles above the layer of liquid and towards the support surface, whereby the bubbles of gas entrained in the layer of liquid are moved closer to the second exposed surface and ruptured.